Method of making castings having chromium alloy surfaces



Patented July 7, 1925.

UNITED STATES- PATENT OFFICE.

WALTEB' H. MITCHELL, OF PHILADELPHIA, PENNSYLVANIA, ASSIGNOB TO B- I. DU PON'I. DE NEHOUBS &, COMPANY, OF WILMINGTON, DELAWARE, A CORPORATION 01 DELAWARE.

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' 'R'o Drawing.

To all whom it may concern:

Be it known that I, WALTER M. MITCHELL, a citizen of the United States, and a resident of Philadelphia, in the county of Philadelphia and State of Pennsylvania, have invented a certain new and useful Method of Making Castings Having Chromium Alloy Surfaces, of which the following is a specification.

This invention relates to the making of ferrous metal castings the surface portions of which are alloyed with chromium, and in articular the invention relates to a met od whereby in the making of such casting, the production of perfectly and completely coated castings is facilitated. In producing the castings the difficulty of having portions not properly treated is sometimes encountered, it being particularly evident at the points of the casting where the washing effect of the molten metal is greatest, as at the point where the gate or runner enters the mold. It is the principal object of the invention to provide a method of the character indicated. The present procedure finds a particular application in the making of castings in accordance with the method disclosed in U. S. application Serial No. 463,565, filed April 22, 1921 by Charles V. Jacobs, entitled Castings having chromium alloy surfaces, and therefore, while not restricting it to practice with that method the invention is here described with more particular relation to that method.

In brief, the Jacobs invention comprises bringing the base metal (for example, steel) during the casting into contact with chromium, this being conveniently accomplished by coating the surfaces of the green mold or cores in which the casting is made, and which correspond with the surfaces of the casting on which the alloy is required, with chromlum in granular condition and associated with a suitable binder (for example, sodium silicate), and baking the mold to fix the coating; wherefore the casting metal flows over and melts or dissolves the coating, giving a casting with the desired chromium alloy surface. Jacobs proposes to use upon all the surfaces to which (the chromium is applied a uniform coating all of the chromium of which is in particles 15 to 4 I mesh, preferably 18 to 30 mesh.

Application filed September 24, 1924. Serial No. 788,458.

The present invention has especially to do with ensuring that during the pouring the chromium coatinggshall 'not be fused and Washed away, leaving areas which are not completely coated, or, indeed are entirely bare and denuded of coating. This denuding action is particularly to be guarded against at those points of the casting where the greatest heat is liberated and where the washing effect of the stream of molten metal is greatest, that is at the points where the gate or runner enters the cavity of the mold.

I have found that this difliculty may be guarded against and overcome bv doublecoating the regions to be guarded, using a double layer of chromium paste, with a finer crushed chromium for the second or exposed layer than that specified by Jacobs for his uniform layer. Thus, the second layer should, according to the invention, be of particles between 80 to 100 mesh, while the lower layer, following the general recommendation of Jacobs, should be 15 to 40 mesh.' The fine material is'more diflicultly fused than the coarser material, and slows down the action of the stream of hot metal which would fuse and Wash away the chromium at the regions in question; while the use of the coarsermaterial gives the quality and depth of alloy surface not feasibly obtainable by the general use of fine grained material. The thickness of the coarse coating may be 4; inch to inchover the ent1re surface and the fine'layer about inch to 4 inch applied over the coarse layer.

My invention may be illustrated in detail by the following example:

A mold for steel casting is prepared in the ordinary way and the surface of this mold While in the green state, (that is, before baking it, as is usual with molds for steel castings) is coatedto the proper depth, for example of an inch, with coarse (15 to 40 mesh) comminuted or granulated chromium mixed with ordinary silicate of soda solution. Sufiicient silicate solution is added to the chromium to form a plastic mass which can be applied with a molding tool evenly all over the mold or core surface it is desired to coat. With a mixture of the right consistency, each square foot of coating of an inch thick will contain about 1.2 lbs. of chromium. -This silicate of soda causes the chromium to adhere to the surface of the sand composing the mold and after baking the mold to a temperature of about GOO-700 F. (that is, the usual temperature of baking molds for steel castings) the metallic chromium is cemented to the surface of the mold so firmly that it can be rubbed or brushed without dislodging it. This general coat is of chromium of 15 to 40 mesh as before stated. This coat having been applied and allowed to set, but before the baking, the second coat is applied to the selected portions, it being in paste form with sodium silicate, and the same method of application, and the same consistency being conveniently used as specified for the general "coat, the depth of this second coat being about inch. For this coat there is used chromium of between 80 and 100 mesh. After the mold has been so prepared, and dried and baked, the casting of themetal is carried out in the usual manner. A pouring temperature of approximately 2650- 2900 F. is usually found suitable for castings, the molds of which are coated with chromium to the depth of as much as A inch.

Castings made in molds having their particular portions protected in the manner indicated will be found to have the desired alloy formed uniformly thereover, those parts which lay near the ate or runner being properly surfaced with alloy as well as the remainder. Castings so produced will have a distinct surface layer of chromium alloy, the chromium content of which is greater than 25%, with the chromium content of the base metal of the casting substantially less (for example, less than 10%).

Although silicate of soda hasbeen particularly referred to as a binder, it being preferred, it is to be understood that other binders such for example as glutrin (a commercial product obtained by concentrat ing waste sulphate liquor), molasses, or similar tacky substances may be used. Irrespective of the particular binder, only sufficient to just accomplish firm adherence of the coating to the mold should be used.

Although straight carbon cast steel has been referred to as the base metal it will be understood that the procedure is also ap-' plicable to steels having additions of other. metals to give them special properties, as chromium, nickel and silicon to eliminate warping, and manganese, tungsten and molybdenum for preventing porosity as disclosed inmy application Serial No. 739,502 filed September 24, 1924 for castings having chromium alloy surfaces, Mitchell Case 1; and also the procedure is applicable to cast 11'011.

While for simplicity merely chromium has been referred to as the coating material it will of course be understood that materials whose primary content is chromium, as fer ro-chromium and the like may place of pure chromium if desired.

Although I have made particular reference to the use of an overlayer of fine chromin-m, as the commonly practicable way of working, it is to be understood that I appreciate that in certain particular circumstances it may be desirable to place the layer of fine below the layer of coarse, or to use, at selected locations, all fine, the general coat of coarse chromium extending over the other coated portions.

I clann:

1. In the production of ferrous metal cast ings having a chromium alloy surface, which production includes coating the mold sur- -face with chromium and a binder, and casting in such mold; the application to a selected portion of the mold of relatively fine chromium, such portion being one where the washing efi'ect durin pouring is excessive,

be used in thereby to prevent .denuding of the coated selected portion during curing.

2.- In the production 0 ferrous metal castings having a chromium alloy surface, which production includes coatin the mold surface with chromium and a bin er, and casting in such mold; the double coating of the mold with a. general layer of relatively coarse chromium and with a layer of relatively fine chromium. a

3. In .the production of ferrous metal castings having a chromium alloy surface, which production includes coatin the mold surface with chromium and a inder, and casting in such mold; the double coating of the mold first with a general layer of relatively coarse chromium and then with an overlayer of relatively finer chromium.

4. In the production of ferrous metal castings having a chromium alloy surface, Which production includes coatin the mold surface with chromium and a Einder, and casting in such mold; the double coating of the mold with a general layer ofchromium of 15 to 40 mesh and an overlayer of chromium of 80 to mesh.

5. In the production of ferrous metal cast ings having a chromium alloy surface, which production includes coating the mold surface with chromium and a binder, and casting in such mold; the double coating of the mold first with a general layer ofchromium of 15 to 40 mesh and then, at those portions "subjected during casting to excessive wash- 

